Electrical apparatus and method



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ELECTRICAL APPARATUS AND METHOD 3 Sheets-Sheet 2 Filed April 26. 1945 March 3l, 1953 v. c. wlLsoN I :rAL 2,633,540

ELECTRICAL APPARATUS AND METHOD vzey Patented Mar. 31, 1953 2,633,540 ELECTRICAL APARA'rUs AND METHOD Volney C. Wilson, Santa Fe, N. Mex., and John A. Simpson, Jr., Chicago, Ill., assignors to the ,United States of America as represented bythe United States Atomic Energy Commission ,Y Application April 26, 1945,-Seria l No. 590,425Y

Our invention relates to a method and apparatus for reducing the insensitive or dead time of a gas filled electricrdischarge tubesuch as, for example, a Geiger-Mller` counter.V More specifically, our invention relates to `a method and electrical circuit foreffecting a reduction inthe natural insensitive or dead time in a tube ofthe Geiger-Mller` type, thus making it responsive to very high counting rates.

Geiger-Mller counters and proportional counters are generally used for detecting` `ionizing events such ,as'caus'ed by the radioactive disintegration'of radioactive'materials. A generally used construction oi' aGeiger-Miillei` counter is one having a pair of concentric electrodes, nainely a small central wire anode anda large cylindrical rcathode also-serving as achamber wall that contains an ionizable medium, such as one or more permanent gases veither with or without an organic vapor. In a counter having an ionizable medium including van organic vapor, the dead time or-insensitive time is ofthe order of -4l second. This dead time occurs'between the time when the initial ionization is produced and the time at which the eld intensityv in the region ofthe anode has recovered sufficiently to detect another ionizing event. A dead-time of this orderfof magnitude is a serious limitation in obtaining accurate data at high counting rates, such as in the case of analyzing the radioactive decay process of materials of short half-life. As a result of an ionizing event, a positive ion space charge or ion sheath forms about the circular wire shaped anode and moves radially outwardly as a cloud to the cylindrical cathode where it is l.

finally collected. The above mentioned limitation of the dead time is considered to exist because of the iinite mobility of this positive ion cloud in its transit towards the cathode, which varies in accordance with the kindof gaseous medium'and electric fields used.

An object of our invention is to provide a method and apparatus for decreasing the natural insensitive or deadtime of a radiation counter tube, such as, for example, a Geiger-Mller tube, to values well below the natural limit of about l(l"` second, more specifically, to as low as .10-5 second or even lower, thus making the radiation counting tube at least about ten times as fast as presently known Geiger-Mller tubes.

A more specific lobjectof our invention is to provide a method and an electrical system for accelerating the removal of the positive ion cloud formed in electric discharge tubes, such as, for example, radiation counter tubes, thereby con- 2,7] claims. (c1. 25o-83.6)

siderably decreasingfthe' insensitive :or dead :time of such tubes.

Inrits broader aspects, .our invention involves the principle of employing an electric iield reversingcircuit for speeding up the collection-"of the positive ion cloud, thus making the radiation counter tube faster. In otherV Words, in'- stead of allowing thepositive ion sheathV which forms around the center wire to move outwardly, the potentials on theY center wire and outer cylinder or cathode of the counter are reversedso that positive ions may 'bey rapidly collectedon the center wire instead of on the--outercylindenginasmuch as the high iieldA atthel center wirerac-v celerates the tions towards the center wire and allows them to be collectedina very short time. namely, in about 10-5 second` orless. Immediately following collection oithe ions by the center wire, normal operating potentials are restored in the counter to place it in condition to vregister the next ionizing event. In this `manner of operation the outer cylinder serves only as a field forming conductor. p

Other objects and advantages 'will become more apparent from thefollowing specification taken with the drawings wherein: Y l f;

Fig. 1 is a schematic circuitgdiagram showing, in block diagram` form, the general components of a Geiger-Mller counter circuit embodying the teachings of our inventionLtogether with certain voltage pulse characteristics;

Fig. 2 isa schematic circuit diagram of a detailed l Geiger-Mller counter circuit, illustrated generallyin Fig. 1, together with the pulse forms to be found at the various tubes;

Fig. 3 is a graph showing instantaneous values of the voltage pulses on the Geiger-Mller-tube plotted against time for two i differenty circuit conditions; and s,

Fig. 4 is-a graph showing the factor of reduc-'- tion of insensitive time .Y

for two counters having diil'erent gaseous compositions. Y .Y

'The following is a detailed theoretical description of the operation of a GeigerLMller counter and of the fprinciplesinvolved in our invention.-

A VCTeiger-Mller counter tube is anelectroni tube, or more precisely, an ion tube. Physically, it is a concentric electrode system in a vacuumtight envelope containing a gas or mixture of gases. As illustrated schematically in Fig. l, the anode 2 is usually a wire shaped central electrode and has a diameter generally less than -2 centimeter; the concentric cylindrical cathode 3 forming the chamber wall is very large relative to the anode. The field intensity is, therefore, inversely proportional to the radial distance from the anode 2 with very high potential gradients existing in the vicinity of the center-wire or anode 2, andaverysmallgradient existing near the cylinder or cathode 3. The gases are chosen for use in such-a ,tube primarily for their ability to produce ions `when a beta particle (B), alpha particle ((1) gamma-,ray (y),l

or photon passes through the volume defined by cathode or cylinder 3. There are, however, a great number of modifying conditions as will appear hereinafter which restricts .severely :the kinds of gases which may be used.

".When an ionizing yevent takes place in the counter volume, the electrons produced are accelerated toward the region of the positive center-wire where, due .to their relatively high energy, .they produce the so-called Townsend avalanche. In the immediate vicinity of the oenterewire, the discharge spreads laterally very rapidlythe time being in the order of 10J! secondl :The mechanism of this lateral discharge lsstill uncertain, ibut. it is most probably due to photons being absorbed by-:gas molecules polarized inthe high field.

VThe 'electrons-are collected on the center Wire in about '10*6 second. As the electrons are collected Von the center wire, a uniform sheath of positiveions is left in the immediate vicinity of the center wire because of the very low mobility of these ions compared with the mobility of electrons. The positive ion sheath immediately begins to move radially outward as a cloud toward the cylindrical cathode-'3 Where it is collected. However, before thepositive ion sheath has left the immediate vicinity of the center Wire, the rleld vexisting between the positive ion sheath and center wire is `reduced to such an extent thatif another ionizing event should -occur no avalanchewouldbe formed. However, after the positiveion cloudhas moved toward the cylindrical.cathode of the order of 1/2 to 2A; of the total radial distance, the field-will be restored toa value that isabove the criticalvalue necessaryto sustain an avalanche -produced by a subsequent ionizing event. The insensitive or dead time, VVtherefore, is the period of the time required for the positive ion sheath to move from the immediate vicinity of the center wire to a position such that the field intensity between the sheath and'center-wire is high enough to support an avalanche. The insensitivetime, therefore, is dependent upon the speed or mobility of the positive ions.

There are, in general, two types of Geiger- Mller `counter gaseous fillings: (l) afilling of one or more inert or permanent gases, such as argon and neon, and (2) an inert and a polyatomic organic gas mixture, such as argon and alcohol vapor, or neon and amyl acetate vapor.

`In alcounter having a permanentgasiilling the positive ions have a high mobility, hence theitirnel requiredfor the positive ionsheath to move out to the criticalv distance is relatively short. 'Thereforethe field intensity at the ,center Wires-returns rapidly to avalue thatv cansupport a new discharge. However, an outstanding disadvantage of this type of counter is that photons and secondary electrons or photoelectrons are formed. These electrons are capable of reinitiating the discharge, thus keeping the counter in continuous discharge. In order to prevent the occurrence of a continuous-discharge, it is necessary to provide an externalcircuit'having a high RC value so as to remove the potential from the center wire following an ionizing event for an interval long enough to stop the discharge. Therefore, While the natural insensitive time of a rare gas counter is extremely low advantage cannot be taken of this low insensitive time because of subsequent production of electrons at the cathode by the arrival of the positive ion cloud. The necessity of a large RC external circuit imposes an unduly long inactive or dead time to the counter, making it unsuitable for detecting ionizing events occurring at a high rate.

Circuits havebeen developed fon example, H. V. Neher and W. W. Harper, Phys,l Rev.49, pp. 940-943, June 15, 1936) Whichquenchfamare gas counter in the minimum of time.` However, even though the Yrare gas ,101.15 have greater mobility than the polyatomic gases, one must reduce the voltage on the counter (below the voltage at which Aan avalanche canbe-initiated) for a sufficiently long time afterthe initiation of a pulse so that oneis sure ,that alithe positive ions have been collected on the. cathode. practice it has been found, that the minimum for this timeY is one or two times l0-4-.se,cond.

In acounter having the lsecond type ofggaseous filling, that is, for example, one containing a permanent gas and a vapor, the ions are polyatomic and, therefore, relatively heavy, hence the positive ion cloud has relatively low mobility and will take an appreciable length of time to move from the vicinity of the center wire to the vicinity of the cathode, that is, it will requirefa time period of the order of 10-4 second, which makes it unsuitable for detecting ionizing events occurring at a high rate without coincidence losses. However, one outstanding advantage of an organic vapor counter isthat practically no secondary electrons or photons are formed,- hence there is no necessity for delaying the restoration of the normal voltage on the centerwire. Therefore, while the positive ion mobility is lower than that of a rare gas counter, ,the insensitive or dead time is shorter because ,of the absence of additional ionization, hence avoiding the necessity of a high external RC circuit for eliminating the effects of secondary electrons and photons.

A counter containing polyatomic gas, such as alcohol, has been called in the literaturea fast counter in contrast to the monor di-atomicgas counter, or rare gas counter, which is called slow because of the relatively long time for which the counter must be made insensitiveto insure that only one count is recorded. This may be a rather unfortunate choice of terms since actually those counters containing p OlyatQmic gases are much slower in completing theremoval of ions from the initial ionizing event thanthe so-called slow counters.

Other difculties encountered in the choice of an atmosphere for a Geiger-Mller counter are the possibilities of producing metastableatoms and negativeions. A vnietastable atomfcanpro.- duce aspurious-countby causinathe. production of an"av`alanche a considerable time after the initial avalanche has occurred. At least a trace of a foreign gas, added to a pure gas counter, is usually sufficient to remove the metastable atoms safely without producing any spurious counts. The addition of a second gas may also produce a counter having a lower starting potential because the energy from an excited level of one type of molecule is great enough to contribute to the ionization of the other type of molecule.

Negative ions have low mobilities. When they are accelerated toward the center-wire, they give up an attached electron which produces an avalanche. This process results in a spurious count. It is essential, therefore, to avoid, if possible, gases containing atoms having high electron aflnities, therefore tending to form negative ions, such as BFa, O2, C1 I, Br, and l-lg. In filling a counter evacuated with a mercury pump system without a liquid air trap, the mercury vapor pressure in the counter will be approximately 0.002 mm. Hg at room temperature. It has yet to be shown that mercury contamination has any pronounced effect on the Geiger-Mller counter action.

The voltage pulse in a fast counter due to an ionizing event is quite different from that in a ".slow counter. In the former case, as the RC may be very low, the pulse front is steep due to electron collection and the tail formed on the end of the voltage pulse will be rather small. However, as pointed out before, a slow counter must have a relatively high RC circuit to insure a faithful count. This results in a pulse which must be sharpened electronically before it can be used satisfactorily for recording unless an electronic quenching circuit is used to reduce this time.

The natural insensitive or dead time associated with each type of Geiger-Mller counter places a severe limitation on its use in obtaining accurate data at high counting rates. Since, in general, the events which are recorded by such counters are purely random, at high counting rates there will be a large number of intervals between two successive events which are less than the insensitive time interval of such counters. This loss of counts at high rates is expressed in an equation in which the insensitive time is the true counting rate is Nt, and the observed rate is Na: the equation is The very best conditions that can be obtained in recording data from a counter with insensitive time are attained when the amplifier responds to the very tiny pulses which may occur immediately after the lapse of the time interval following each ionizing event, unless the counter is operated just below the Geiger threshold. In the latter case, the sheath does not fully develop,

and the pulses are not independent of the kind of ionizing event. The natural lower limit on the insensitive time of a Geiger-Mller counter therefore limits its use in obtaining accurate data at high counting rates. The positive ion space charge in fast counters unduly prolongs the time that the field is below the critical value necessary for supporting a new avalanche. If a high mobility cloud could be formed, this time would be considerably reduced. Such a cloud is encountered `in a permanent gas mixture of two light gases, as indicated previously, but to date no method is known for preventing secondary electrons and photons from forming in such a counter.' The mobilities of all suitable polyatomic gases are of the same order of magnitude and' counters'should be filled to pressures of the order of 10 cm. I-Ig to 'be ecient. If an attempt is made to use polyatomic gases which have a very The ion space charge is, therefore, the con-` trolling and limiting factor in reducing the counter insensitive time.

Another mode of operation of a Geiger-Mller counter is evidently necessary if progress is to be made in reducing the insensitive time. total collection time for electrons on the centerwire of an organic vapor-noble gas counter is less than 10"6 second, and most of the electrons are collected in about this time in a slow counter. Therefore, after the accompanying initial drop in center-wire potential, use is not made of the subsequent cycle so far as recording the event is concerned.

In accordance with our invention, instead of allowing the positive ion sheath which forms around the center-wire to move outwardly towards the cathode, these ions are almost immediately collected on the center-wire. This can be accomplished by an electronic circuit that reverses the potentials on the center-wire and cylinder immediately after the ionizing event and maintains the potentials reversed long enough to insure collection. Since the ions are formed only in the immediate vicinity of the wire, and will therefore be in a high fieldwhich accelerates them rapidly toward the center-wire, the collection time will be Very short, that is less than 10-5 second. As soon as allions are. collected, the counter potentials are immediately restored to their normal operating values to be ready for the next` ionizing event initiated by the radiations being detected. Thus the-outer cylinder serves only as a field forming conductor.

By making the center-wire negative, the production of photo-electrons becomes negligible. Photo-electrons are produced at the cathode of the electrode system which in this case is the center-wire. Therefore, this photo-effect is reduced by a factor approximately equal to the ratio of the wire radius to the cathode radius. It should be noted that tungsten, which is widely used for center-wires, has a work function of about 4.5 e.v.

The behavior of positive ions near the negative center-wire for potentials below the inverse Geiger region is probably quite complex and does not lend itself to an exact solution. It is desirable to know under what conditions the positive ions may acquire enough energy to produce secondary electrons by field emission at the wire. If the ions are polyatomic, they may be polarized near the wire and change the effective mean-free-path in that region.

The avalanche process at the negative wire hasv manyl characteristics which differ from lthe wellknown positive center-wire avalanche. If a positive ion approaches the center-wire and causes the ejection of an electron by eld emission, this electron will travel toward the outer cylinder. The eld is decreasing as 1/1 where r is the radial distance from the center-wire. By the time electron avalanche production would be able to increase to very large values, as in an increasing eld, the avalanche head has already entered a low intensity field region. Townsends coefficient a, which 'is a measure of the number of The assegno,

ionpai-rs formed by an electron vin one centimeter o'f-i-ts path, depends upon the field intensity and the pressure. Since a is decreasing almost exponentially as the 'avalanche head moves into a region of low E/p, whereE the eld strength and p1: the pressure `of the gas, the production will break olf fairly sharply. The .field is further distorted and reduced by the positive space charge left-behind the-avalanche. The current flow of electrons, therefore, increases rapidly to a vaule which is constant for most ofthe distance-out to the cylinder. If many electrons initiate a large number oavalanches, the positive-space charge may be suiiicient momentarily to choke oil further avalanche production-until a large lportion of the positive lions have been collected. The process -thenrrepeats producing an oscillatory discharge` The negative ions play an important part if a great number exist when the counter potentials are reversed, for then the negativev ions must travel out toward the cylinder. However, if only afew such ions are formed, they will cause no diniculty because they will not materially distort the field distribution. They will not lose their electrons in a region where a new avalanche could possibly star-t because the field `adjacent to the outer cylinder` is rvery low.

These general considerations indicate that the new -mode of counter operation proposed 'can be successful if the proper inverted region is selected by maintaining the magnitude of the reversing voltage below a predetermined value, generally'less than 1/3 o1 the' normal operating voltage. For example, for a normal operating voltage of S to 900 -volts a reverse potential of Y i150 lto -186 volts has been found to be satisfactory,

In recording an ionizing event the shape and entire duration of the pulse developed across the counter in normal operation is of no particular importance, the steep pulse front which is produced in the order of 1 microsecond by the high mobility electr-ons arriving at the center-wire being suflicient for counting purposes. As described previously, the positive ions which remainfas a result of the coun-ter avalanche are left in the form of a sheath or space charge around the center-wire and begin to migrate to the outer electrode. In order to -collect positive ions rapidly at the center-wire it is necessary to effect triggering of the circuit almost immediately upon the occurrence of the ionizing event. Itis desirable, therefore, to use the rst 1 cr 2 volt change of counter potential which is produced by the electroncollecticn and slight initial motion of the positive ion sheath, to detect an ionizing event and subsequently record a count. Some mechanism must be provided for transforming this first `portion of the voltage pulse int-o a-large pulse of short duration suitable for triggering an electronic circuit that is capable of reversing the potential of the center-wire with respect to the outer electrode. The total time forcarrying out this potential reversing process must be short enough so that the positive ions Will not have an opportunity to move any appreciable distance from the vicinity of the centerwire.

A suitable electronic methodfor reversing these potentials in accordance with our invention is rst to amplify the small pulse front in such a way that the high frequency components'of the pulse front are preserved so as to give a sharp trurgering pulse. This triggering pulse can then be used toactuate an electronic circuitcapable of producing a rectangular Wave-whose Width will `ultimately determine the time of positive ion collection. This newly formed wave may then be amplied and inverted so as to produce a very largepulse of the same time width capable of inverting the .electrode potentials.

Fig. l, is a schematic drawing showing in block tive vpulse is applied to an amplifier and shutter circuit. The pulse is amplified and made to `actuate a trigger pair Vfor shutter" which produces a rectangular wave of any desired width t1. Since it is apparent that any subsequent Y reversal of counter potentials also induces pulses in the aniplien it is necessary that the amplifier be shut down from normal operation, that is, made non-responsive to incoming signals, for an interval of time long enough for the positive ion collection to take place. This is accomplished by feeding this rectangular wave of Width t1 back into the amplifier in such a way that it temporarily shuts off or blocks the amplifier. The saine triggering pulse also actuates a second trigger pair for reversing that produces another rectangular wave Whose width tz determines the ion collection time. This pulse is amplified in a pulse circuit which actually produces the inversion of potentials. Referring.

to the resistance ynetwork in Fig. l; normally no current flows through resistance Rb. Hence, the potential at the point A will normally be at the same value as the positive (-1-) power supply high voltage V2. The high voltage supply has a potential Vi-l-Vz, but a portion of this potential is negative with respect to ground, namely V1. Since the counter tube l operates between ground and the point A, its center-wire 2 assumes the potential difference V2, normally. Now when a large current flows through Re as the result of an ionizing event for a time determined by this second rectangular wave ci width t2, the potential at point A drops, therefore, the center wire potential becomes negative with respect to the outer electrode of the counter for the period t2, provided the amplitude of the Wave form shown in Fig. 3 is greater than V2. At the end of the interval t2 determined by the second triggering rectangular wave, the potential at A again returns to potential V2 with respect to ground. The counter is now ready to record another ionizing event, but the amplifier is not. The amplifier becomes sensitive after a time t1, and thenthe entire system is again sensitive."

Fig. 2 shows in detail the circuit diagram illustrated generally in Fig. 1. Referring more particularly to Fig. 2 and comparing vit with Fig. 1, tubes Ti T2, and T3 comprise theA amplifier and shutter, tubes T1 and T2 being amplifiers and T3 being an amplier and shutter; tubes T4 and Tcomprise the trigger pair for shutter producing the shutter pulse of duration t1; tubes Ts and Tv comprise the trigger pair for reversing producing the pulse of duration t2; and tubes Ts and T9 comprise the pulse circuit. The network LSRSRzv in the plate supply of tube Tg corresponds to Rb in Fig. 1. In each of the tubes Ti to T8, inclusive, the electrodes represent in succession starting from the cathodeas follows:

assen@ fTs, T4 and Ts andappearing at the output of tubes T5, T1, Ta and Ts. These pulses are not drawn to scale.

Assume that an alpha, beta, or gamma event produces ionization inside the counter tube I. A number of ions are formed. The mobility of v the electrons is about l03 times that of the posielectrons arrive at the center-wire, the discharge spreads laterally and around the wire in both directions uniformly Within about 10-7 second. This results in the formation of a rather uniform positive ion sheath around the center-wire,

"which reduces the eld intensity between the center-wire 'and the 'cathode to such an extent that if another ionizing event were to occur at that instant no avalanche would take place. The arrival of the electrons at the center-wire results in the beginning of the irst portion of the normal Geiger counter" pulse. If the counter were allowed to operate in the normal fashion, it would produce a pulse that Would have the shape of the total pulse Va shown in dotted lines associated with the voltage pulse Ve. However, it

" is only the iirst portion Vb or (rst few volts change) of this pulse that is required to change the potential of the grid of the non-conduct- 1 ing tube T1 so that it may become momentarily conducting so as to produce a sharp negative pulse V2 on the control grid of the normally conducting tube Tz. It should be noted that the bias potential of tube T1 is adjusted so that only f positive pulses are amplified. The tube T2 is `operated well up in its characteristic curve and therefore will produce a highly amplified positive pulse V3 lat the `grid of `the normally non-con-v ducting tube T3. By utilizing inductances and differentiating networks such as L2 and CgRu, it is possible to make the pulse V3 extremelysharp. Tube T3 serves two purposes rst, it inverts the signal pulse producing a negative pulse V4 at the grid of the normally conducting tube T4, and

. second,vv tube T3 has its suppressor grid adjusted so that this grid may make the tube inoperative when the large negative rectangular wave of timeor width t1 appears thereon through condenser Ci. Such a rectangular wave of width t1'` is produced by the biased trigger Apair circuit comprising normally conducting tube Ti and normally non-conducting tube T5, as follows.

' As a result of voltage pulse V4 the control grid of tube T4 becomes more negative causing the plate voltage of tube T4 to rise. The grid voltage I of tube Ts therefore becomes more positive caus-l ing a drop in plate voltage thereof which is fed y back to the control grid of tube T4 making it even more negative and causing the plate voltage of y tube T4 and grid voltage of tube T5 to rise further.

Hence, Ythe cycle repeats over and over again until finally tube T4 becomes non-conducting and tube T5 becomes conducting resulting in the first' vvertical portion and the beginning-of the horizontal portion of the rectangular wave. This new condition is an unstable one because the charge on condensers C5 and Cs'leaksoifthrough resistor R13 and the charge on condenser C1 leaks off through resistor Rie. Hence, at the end .of a time period .t1 (determined by the various parameters'and value of variable resistance R13) Vtube T4 is restored to its conducting condition,

and tube T5 to its non-conducting condition thus 4giving the second vertical portion of rthe rectangular wave. The rectangular Wave isfed to the suppressor grid of tube T3 making tube T3 non-conducting, hence blocking 01T the amplifier against further translation of pulses. Tube T-a thus acts as a shutter forthe ampli-her sovtliat -after an ionizing'event takesplace, theampli- -iier will not pass a subsequent pulse onto tube lTi y for a time t1 which is the width of the rectangular Wave produced bythe trigger tubes T4 and T5. This will insure that' any disturbances occurring during this time interval on the grid of tube lT1 will not be counted or'trigger the reversingcircuit. Simultaneously the rectangular wave"is impressed on aV scaling and recording circuit (not shown) which records the ionizing events.

Tubes Ts and T7 constitute a second triggerA pair similar'to tubes T4 and Ts, that is, tube Ts is -normally conducting and tube T7 is normally nonconduoting. Tubes T and T7 are triggered bythe negative portion of voltage pulse V produced on the control grid of tube Te by diiferentiatingfpulse V5 by circuit'Ris C9.v Trigger pair tubes T and T'z also produce a rectangular wave Vff'n'th'e plate of tube T'Vof 'duration t2. Periodtz maybe adjusted to' the desired collection time for the positive ions by varying "resistance Risffthat'is, by increasing Ris, t2 becomes longer. This vpulse is inverted and ampliiied by normally lconducting tube Ta The pulse Vs so formed at theplate of tube Ta then appears on the'controlgrid of tube T9." Tube Tg is biased so that it-V is normally non-conductingand requires a large positive pulse on its' grid to` lmake it highly conducting. The large positive pulse Vs resultsin: the production of a much larger` negative pulse Vifat'the output of tube T9, that is, approximatelyequ'al to the potential Vi-l-Vz, thus it brings the plate of tube T9 to almost vcathode potential. But this drop in potential caused by current iiow through circuit R27-Rs-Ls is greater than the potential existing between the center electrode v2 `and the cathode 3 of the counter. Therefora-the counter `potentials are inverted by this pulse for a time t2 to a magnitude of approximatelyV1v volts. The pulse V9, which is illustrated foreshortened,

now makes itvmore apparent'why the -amplier must be made inoperative for an interval of time t1. It will be noted that the counter I and resistance R form an RC network with a very small time constant. This differentiates the pulse V9 at'the control grid of the tube T1, but differentiating of pulse V9 means that both a negative and positive sharp pulse Vc appears at the control grid of tube T1. Tube T1 is not sensitive to a negative pulse, but is sensitive to a positive pulse which is blocked from translation by Vthe negatively biased shutter tube T3 since the amplier is shut down for a time ti which is greater than the time t2. Theoretically satisfactory operation may be' obtained by making t1=t2, that is, by using therectangular wave form t1 instead of t2 for reversing. Practically, however, there will always be a tendency for lvoltage pulses to appear at tubel T1, understhis critical operating condition, therefore; ideal operation is not obtained.

Iherewouldhei no advantage' in this circuit if Vthetailportion of pulse Vrwere so great that ""tliecount'erdid lnotv actua-llyreturn to its normal *operating condition almost immediately after the'tlme'tz had elapsed. The `removal of'thiS tail'k is" accomplished by acircuit LSRs which disj'sipates the electromagneticenergyinthe plate Yclrcuitupon"thefreturrr'oftube Tato vnormal cuto conditions. The tail is removed by`l the corl'rect choice of resistance Ri 4and inductance Values'other than the correct val-uesffor RS and Lrproduce long characteristictails Whichare undesirable.

'The"rollcwing table illustrates a Set of op- 'ierablevaluesforfthe various circuit elements shown lin* Fig.' 2. itbeinglndica'ted"that other values-may also lue-chosen to give satisfactory l vlapel-ation;- 1012011 mfd.

382:50() ohms H3225 ,00.0 ohms Reer-2500 Ohms the'plate of tube T9 and on .the anode Wirel2in the event vthatY the cir-cuitQLsRswereomitted entirely. It will be noted that -a long -tailoi the order of magnitudev of. 20G volts in height Willbe formed .causingv an unusual' delay in :the-restoration'ofthe plate voltage to its normal value V2. The 'potential at the counter-cathode is indicated by Vet. On the other hand by using Rs .-andiLs in the circuit and by choosing correct valuestherefore, a 'voltage pulse Vac appears on the anode wire 2 andY a voltage pulsev Vita appears ait the cathode 3. It will he seen that practically no tail appears on pulse V90 thus showing :that the stabilizing circuit (or so-called LRC including RLS and Lsor its equivalent 'is almost indispensable for proper operation of the reversing circuit.

Fig. -4is a graph in which the ordinate `anf .ir 011 represents the'ratio of insensitive-timev with the reversing circuit inoperative (a nii) to the insensitive time with the vrevers-ing vcircuit opera- .RSL-2500 ohms' R6-:251300 AohmsY Rie-:0.1 meg.

VThe'requirements for the design` of the above described electronic circuit -to produce the ran- `dem reversals -at a very high average -rate with precision are stringent and. may he: summarized ais-follows:

1. The input to the amplifying device Ymust be responsive to the iirst few voltsdrop vof the steep jpulsefront and must be, insensitive tothe process or four Volts can be tolerated after the collection `.time is over otherwise at high counting rates,

the eiectve operating potential will he changed. 5'. Accurate control o'f the time and extentof Y'the reversalv of field-must be possible over a Wide range to accommodate all types oi counters.

'6. Al'lsup'plypotentials must be constant under extreme liuctuations of counting rates.

`lig. 3 shows similar but enlarged curves representing vthe voltage pulses l.appearing lon the *plate of 'tube T9. The pulse' Vga is obtained on The ahscissa tive (fr on) aonthe resolving time is'ahout lil-4 second. On the other hand, when the Vreversing circuit is used the optimum vval-neef (ir-on) may be of the order of 2x10*5 second for the alcohol-argon counter. This optimum Value is indicated by the peaks of the curve, since increasingY values of the'ordinate represent decreasing values of the insensitive time. Thus for a counter having an alcohol- Yargon mixture, it is seen that for obtaining an optimum insensitive time, namely one in which o'Off O'OD =about 5 it is: necessary Athat .the ratio be vabout .15. Stated diierently, in order'to 10btain optimum (minimi-nn) values of insensitive time, the ratio of vshould be less than .3 for the ltwo counters illustrated. It should also be noted that the proper value of reversing potentials to 'be used for obtaining the optimum results, that is, for obtaining the greatest decrease in insensitive time is dependent upon many variables such vas, for example, the particular gaseous mixture used, the time of collection (that is, the width of the reversing pulse and the shutter pulse width), etc. 'insensitive times of a value of 2x10-5 second vand lower have been readily obtainable by the labove described reversing scheme. JOperation of the system indicates that much lower val-ues, of the order of iiior Ilower 4may he obtained 13 with better operating conditions. In plotting the values shown in Fig. 4, the shutter pulse t1 was set at l.5 l*5 second, and with this adjustment the new insensitive time is about *2x10*5 second. The time t2 was varied to 2.5 -6 second.

It will be seen therefore that we have provided versing scheme by way of illustration lhas been described as useful for reducing the insensitive time of a Geiger-Mller tube, it should be noted that it is likewise useful for reduction of insensi- 'tive time for other electrical discharge tubes.

, It should also be noted that while a specific 'circuit has been described in detail to illustrate the principles of our invention, other equivalent 'circuits for accomplishing the same results may 'be readily suggested to those skilled in the art Aafter having had the benefit of the teachings of ,the present specification which circuits, however, come within the general spirit of our in- .vention. For this reason, our invention should ,no-t be restricted except as set forth in the following claims.

We claim:

1. The method of operating a gas-'lled Geiger- 'AMller counter tube comprising applying oper- Aating potentials to the electrodes thereof, re-

versing the polarity of said electrodes in response ,to initiation of ionization in said tube as the result of entry of a radiation impulse therein, and `restoring said potentials to their initial operating value Within a period of less than 10-4 second.

2. The method of operating gas-filled Geiger Miiller counter tube having a pair of electrodes, vone of said electrodes being an anode and the other a cathode, comprising applying operating potentials to said tube, a positive potential being applied to the anode and a negative potential being applied to the cathode, reversing said po- -tentials as the result of initiation of an ionizing event, thereby rapidly collecting positive ions on said anode electrode, and rapidly restoring said v potentials to their initial operating value after substantial collection of said positive ions.

3. The method of operating a gas-filled Geiger- .Mller counter tube having a central electrode serving as an anode and a surrounding cylin- "drical cathode, comprising applying operating Ipotentials to said tube, reversing said potentials 'as the result of initiation of an ionizing event, thereby rapidly collecting positive ions on said central electrode, and rapidly restoring said potentials to their initial operating value Within .a period of the order of 10*5 second following said reversal.

4. The method of operating a gas-filled Geiger- Mller tube having a pair of spaced apart elec-` trodes, one of said elec-trodes being an anode and the other a cathode, comprising developing I positive ion sheath in the vicinity of said anode as the result of an ionizing event, momentarily applying a negative potential to said anodeV thereby collecting saidl positive ion sheath by countersystem including a Vgas-filled Geiger- Mller counter tube and an amplifying tube for vamplifying the eects of ionizing events in said counter tube comprising applying operating potentials to the electrodes of said counter tube to develop a ow of electrons in response to an ionizing event, developing a pulse of predetermined time duration in response to said Iiow of electrons, and applying said pulse to said amplier to block said amplifier while simultaneously employing said pulse to effect reversal of potengtials on said counter tube electrodes to effect rapid collection of positive ions formed therein, and thereafter rapidly restoring said reversed potentials to their initial operating value so as to condition the counter tube for a subsequent ionizing event.

6. The method of operating a Geiger-Mller counter system including a gas-filled Geiger- Mller counter tube and an amplifying tube for amplifying the effects of ionizing events in said counter tube comprising applying operating potentials to the electrodes of said counter tube to develop a flow of electrons in response to an ionizing event, developing a pulse of predetermined time duration from said flow of electrons, and applying said pulse to said amplifier to block `said amplifier, substantially simultaneously developing a second pulse of shorter duration than said rst pulse and utilizing said second pulse to effect reversal of potentials on said counter tube to effect rapid collection of positive ions, and thereafter rapidly restoring said reversed potentials to their initial operating value.

7. The method of operating a Geiger-Mller counter system including a gas-filled Geiger Mller` counter tube and an amplifying tube for amplifying the eifects of ionizing events in said counter tube comprising applying operating potentials to the electrodes of said counter tube to develop a ow of electrons in response to an ionizing event, developing a first voltage pulse of predetermined time duration in response to said ow of electrons, and applying said pulse to said amplifier to block said amplifier, substantially simultaneously developing a second voltage pulse of shorter duration than said first pulse and utilizing said second pulse to effect reversal of potentials on said counter tube to effect rapid Vcollection ofpositive ions, and thereafter rapidly restoring said reversed potentials to their initial operating value, said voltage pulses being of substantially rectangular wave form and having time durations of less than 10-4 second. 8. The method of operating a Geiger-Mller j counting system including a gas-filled dual electrode Geiger-Mller tube, an amplifier circuit including an amplifying shutter tube and a normally non-conducting pulse amplifier tube, comprising applying operating potentials to the electrodes of said counter tube to develop a now of electrons in response -to an ionizing event, developing a first voltage pulse in response to said rflow of electrons of time duration less than the transit time of the positive ion space charge between -the electrodes of said counter tube, apply- Q ing said pulse to said amplifying and shutter tube to block it against response to subsesaid anode, and conditioning said tube for reionization within a time Abetween 10-4-and l05 f second.

quent incoming signals, substantially simultaneously developing a second voltage pulse of `shorter time' duration-'than said vfirst pulse and utilizing said second pulse to make said pulse amphfier tube conducting-and therebyeffect-reversal ofpotentials on: s'aidcounter electrodes rfor -a-ipenoriVv equal to theduraton `of said Second Pulse.

9. The methodofopcratinsa gas-llcd :Geiger- `lViiiller tube having `a central .aIlOe and a surrounding-cylindrical .cathode comprising alternately depressing-the anode potential to avalue less than the cathodepotential for periods of the order of loes second in response-to ionizing events to cause collection rof the positive ion space charge by said central anode, and there- .ater restoring said anode potential to `its initial value after each reversal so as to condition the tubepfor a subsequent ionizing event.

10. In combination with a Vgas-plied selfguenching VGeigreiwltiiiller tube having a central wire vanode and a surrounding cylindrical cathode, electric means responsive to the initiation of l ionizingl events tocdBPi-ess the anode potential to v a value less-than the cathode potential for periods of time substantially less than the inter-electrode transit period of `positive ions formed therein.

11.111 combination with a gas-plied Geiger- Mller tube compr-isinga center Wire serving as an vanode and a surrounding concentric cylindrical cathode, means for applying operating po- .tentials to saidelectrodes, means responsive to theintial portion .of an ionization pulse front to reverse-*said operating potentials for a period of the order of 10-5 second to effect rapid collection of ions comprising the positive ion sheath formedv as the result -of said ionization pulse.

1.2. A radiation counting system comprising a gas-iilled Geiger-Mller counter tube having an anode and: cathode, means for applying operating potentials thereto, amplifying means responsive to the rst few volts change of a pu-lse front resulting from an ionizing event for translating an empl-ined pulse for recording purposes, and means also responsive to said pulse front -for effecting reversal of said operating potentials on said countfor recording purposes, means to develop a rectangular voltage wave having a time duration substantially less than the inter-electrode transit timerforipositive Vions developed in said counter--tubel asthe resultof an ionizing event, means for applying said Vrectangular voltage wave to said amplifying means to block it, and means controlled Vby said Irectangular wave for substantially simultaneously reversing said counterl operating potentials `for a period less than the duration of said rectangular wave for effecting rapid collection of positive ions in said counter tube to reduce the insensitive time thereof.

14. A radiation counting system comprising a gas-filled Geiger-Mller lcounter tube havingv an anode-and cathode, means for applying operating Y potentials to said electrodes, amplifying means responsive to a small initial portion of an ionizing event pulse Afor translating a voltage pulse for recording purposes, a trigger-pair of electric discharge tubes interconnected so that one tube is conducting -While the other is non-.conducting to .develop a rectangular voltage-Wavehaving a .time

, duration substantially:lessfthan the inter-electrode transit time vfor positive ions `developed in '-16 said counter tube as the result of an ionizing event, means for applying said rectangular Wave to said amplifying means for blocking thereof and means controlled by said rectangular wave for substantially simultaneously reversing said counter operating potentials for a period less than the duration of said rectangular wave for effecting rapid collection of positive ions in said counter tube to reduce the insensitive time thereof.

l5. A radiation counting system comprising a gas-nlled Geiger-Mller counter tube having an anode and cathode, means for applying operating potentials to said electrodes, amplifying means responsive to a small initial portion of an ionizing event pulse for translating a voltage pulse for recording purposes, a trigger-pair of electric discharge tubes interconnected so that one tube is conducting while the other is non-conducting to develop a rectangular voltage Wave having a time duration substantially less than the interelectrode transit time for positive ions developed in said counter tube as the result of an ionizing event, a second trigger pair of electric discharge tubes responsive to said rectangular Wave to develop substantially simultaneously a second rectangular voltage wave having a time duration less than that of said iirst rectangular wave to effect reversal of said counter operating voltages for a period of time equal to the Width of said second rectangular Wave to effect speedy collection of positive ions and thereby reduce the counter insensitive time.

16. A radiation counting system comprising a gas-filled Geiger-Mller counter tube having an anode and cathode, means for applying operating potentials to said electrodes,l amplifying means responsive to a small initial portion `of an ionizing event pulse lfor translating a voltage pulse for recording purposes, means to develop a rectangular voltage Wave having a time duration substantially less than the inter-electrode transit time for positive ions developed in said counter tube as the result of an ionizing event, means for applying said rectangular voltage Wave to said amplifying means to block it, and. means controlled by said rectangular voltage Wave to generate a second rectangular voltage Wave of identical starting time but of shorter duration than the iirst rectangular voltage Wave, and means controlled by said second rectangular voltage wave for reversing said counter operating potentials for a period equal to the duration of said second rectangular wave for effecting rapid collection of positive ions in said counter tube to reduce the insensitive time thereof, said last named means including -a normally non-conducting pulse amplifying tube having an anodeconnected with said counter anode, said .pulseamplifying tube being -controlled by ,said second rectangular voltage wave so as to become conducting and effect depression ofvits anode potential below the counter tube cathode potential.

17. A radiation counting system comprising a gas-nllcd Geiger-Mller counter tube having an anode and cathode, potential `means for applying operating potentials thereto, amplifying means responsive vto the rst few voltschange of a voltage pulse front resulting from .an ionizing event for translating an amplied voitagegpulse for recording purposes, -and'ineans also responsive to said -pulscfront for eiiecting reversal of Y said operating potentials onrsaid counter for a period V.of less than Mft/second to eiTect rapid collection of the positive ion sheath developedin 7s. the weiteres theresult 0f Said ionizing' event.

17 said vlast named means including an inductance and 'resistance network connected in series with said potential means and said anode and having such values as to substantially eliminate pulse tail formation at the anode.

18. A radiation counting system comprising a gas-filled `Greiger-iiller 4counter tube having an anode and cathode, potential means for applying operating potentials to said electrodes, amplifying means responsive to a small initial portion of an ionizing event 'pulsefor translatingja voltage.

pulse for recording purposes, 'means tode'velcp of, said last named'means includinganinductance and resistance network -connected -in series with said potential means'and' saidano'de andf vhavingv such valuesas to substantially eliminate pulse tail formation at the anode. Y*12). A radiation `counting 'system comprising a gs-iilled Geiger-Mller counter tubev having 'fan anode and cathode, means for-applying operating potentials to said electrodes, amplifying means responsive to a small initial portion of an ionizing event pulse for translating a voltage pulse for recording purposes, pulse-forming means to develop a rectangular voltage Wave having a time duration substantially less than the inter-electrode transit time for positive ions developed in said counter tube as the result of an ionizing event, means for applying said rectangular voltage Wave to said amplifying potential-reversing means to block it, and means controlled by said rectangular voltage wave for substantially simultaneously reversing said counter operating potentials for a period less than the duration of said rectangular Wave for effecting rapid collection of positive ions in said counter tube to reduce the insensitive time thereof, each of said pulse-forming and potential-reversing means comprising a trigger pair of electric discharge tubes interconnected so as to become alternately conducting, the rst tube of said trigger pairs being normally conducting. ,Lf

' 20. A radiation counting system comprising a gas-filled Geiger-Mller counter tube having an anode and cathode, means for applying operating potentials to said electrodes, amplifying means responsive to a small initial portion of an ionizing event pulse for translating a voltage pulse for recording purposes, a trigger-pair of electric discharge tubes interconnected so that one tube is conducting while the other is nonconducting to develop a rectangular voltage wave having a time duration substantially less than the inter-electrode transit time for positive ions developed in said counter tube as the result of an ionizing event, said amplifying means including a normally non-conducting shutter tube becoming conducting in response to said initial portion of said ionizing event and including a suppressor grid, means for applying said rectangular voltage wave to said suppressor grid to make the shutter tube non-conductive and block said amplifying means so as not to translate further incoming pulses for thetime duration of said rectangular voltage wave, and means controlled by said rectangular voltage Wave for Asubstantially simultaneously reversing said counter operating potentials for a period less than the duration of said rectangular voltage wavefor effecting rapid collection o f positive ions in said countertube to reduce the insensitive time thereof. f v

2l. The method of operating a gas-lilled Geiger-Mller tube including an anode and cathode that comprises applying operating potentials to said anodeand cathode, then depressingthe anode potentialto a value that is negative Withrespeottothe cathode in response to an ionizing event so as to effect rapid collection, of positive ions, and iinally'restoring the potentials totheir originaloperating values.

i 22. The method of operating Va gas-filled Geiger-Mller tube including an anode and cathodethat comprises applying operating potentials to said anodeand cathode, then depressing the anode potential to a value that is negative with respect to the cathode and less than one-third of the anode operating potential in response to an ionizing event so as to effect rapid collection of positive ions, and finally restoring the potentials to their original operatingvalues within a period less than 11074 'second `from the initiation of said ionizing event.

23. In the operation of a radiation-responsive tube used for, detecting ionizing events occurring at a very rapid rate, a method of dispelling an vion sheath surrounding an electrode having a potential of the same polarity, comprising rapidly reversing the potential of said electrode in response to the initial portion of an ionization discharge, thereby rapidly collecting positive ions, and thereafter immediately restoring said potentials to their initial value.

24. In the operation of a radiation-responsive tube, a method of dispelling an ion sheath surrounding an electrode having a potential of the same polarity, comprising the steps of rapidly reversing the polarity of the potential of said electrode in response to the initial portion of an ionizing event, thereby rapidly collecting positive ions, and restoring said potential to its initial value within a period less than 10-4 second.

25. Apparatus for reversing the polarity of a voltage applied to an electric element having two terminals comprising, in combination, a source of direct voltage having a positive terminal, a negative terminal, and a tap having a potential intermediate between the potentials of said terminals, an electrical impedance connected to said positive terminal, an electron discharge tube having a cathode connected to said negative terminal, an anode connected to said electrical impedance, and a control electrode, means for connecting the junction point of said anode and said electrical impedance to one terminal of said electrical element and means for connecting said tap on said voltage source to the other terminal of said electrical element, and means for impressing upon said control electrode a voltage adapted to change the conduction between said cathode and said anode so that in the absence of said voltage on said control electrode, the anode is positive with respect to the tap on the source of voltage, but upon the appearance of the voltage upon the control electrode, the anode is negative with respect to the tap upon the source of voltage.

26. Apparatus of the class described comprisacamo 19 ing, in combination, a gas-lled` discharge tube having an anode and a cathode, ank amplifier adapted to amplify voltage pulses, means for converting the commencement of a discharge in said discharge tube into a voltage pulse adapted to actuate said amplifier, pulse-forming means adapted to produce a voltage pulseof xed duration shorter than the deionization time of-said discharge tube, means for causing the last said pulse to deactivate said amplier during the duration thereof, and-means responsiveY to theA commencement of a discharge in said discharge tube for substantially simultaneously reversingy the polarity of the potential applied to said discharge tube so that' collection` of rions therein is* accelerated.

27. Radioactivity measurement apparatus comprising, in combination, a Geiger-Mller tube having an anode and a cathode,` a directvvoltage source having a positive terminal, a negative terminal and a tap intermediate between said terminals, an electron discharge tube-having a.- cathode connected to said' negative'v terminal land having an anode connected to said positive terminal through an electrical impedance, the anode of said Geiger-Mller tube beingr connected tothe junction point between saidanode of the electron.dischargeldevice and said electrical impedancc, andthe cathode of saidA Geiger-Mller tube being connected to said tap onthe voltage source, said: electron discharge device having a control electrode adapted to changer-the conduc- 20 tion inv said electron discharge device so that thel polarity7- of the voltage appearingV between the electrodesV of said Geiger-Mller tube varies'in response to variation ofthe potential of said control electrode.

l`VOLNEY C. WILSON.

JOHN" A. SIMPSON, Jn.-

REFERENCES CITED The following references are of'recordin the file: of this, patent:

UNITED STATES. PATENTS Number Name Date 2,071,759 Minorsky Feb. 23,1937 2,141,343 Gampbell Aug. 12; 1938- 2,224,832 Pfister Dec. 10, 1940 OTHER REFERENCES Reduction of; the` Natural Insensitivc- Time' in G-M Counters,l SimpsomJr. (Recd. February- 

